In many systems, a pair of shafts normally are driven at a selected speed or speeds. However, should there be a system failure such as a failure of a power drive unit for one of the shafts, or a failure of the shaft itself, a large torque spike can be imparted to the system because of the large relative speed between the shafts and the rapid deceleration that will occur when system stops are encountered. As a result, such torque spikes make it necessary for components to be upsized in many systems.
For instance, on many aircraft actuation systems, a primary drive shaft is used to connect the power drive unit to the flap actuators. A secondary drive shaft is normally connected to the power drive unit through a lost motion device that enables detection of a broken primary drive shaft, but prevents torque from being carried by the secondary drive shaft under normal operating conditions. With such systems, a significant problem occurs when a primary drive shaft fails under aiding air loads.
In particular, aiding air loads cause the flap and actuators to rapidly accelerate until the extreme of lost motion is reached. When this occurs and stops are encountered, the rapid deceleration caused thereby will impart a large torque spike to the system. In order to avoid the necessity of upsizing the components of the flap system, it is necessary to minimize the torque spike by limiting the relative speed between the secondary drive shaft and the power drive unit.
In a specific system for aircraft, the flaps are normally driven by two power drive units which each include an electric motor connected by means of a flexible shaft to one side of a torque sensor. If an electric motor should fail, the torque sensor will connect the flexible shafts to allow the remaining operable motor to drive the entire system and will send a signal to the main controller signaling a malfunction. Unfortunately, it is generally recognized that several significant problems can arise in from such systems.
Specifically, lost motion is required in the torque sensor to allow the flex shafts to remain unloaded. Because of the lost motion, the two sides of the system, i.e., the primary and secondary load paths, obtain a large relative speed in the event of a motor failure under load. As a result, system components can be severely damaged when the available lost motion is used up, e.g., stop and flex shaft failures can be experienced from inertial loads, due to the large relative speed.
Among the attempts to provide shaft relative speed limiting devices is that disclosed in Clark U.S. Pat. No. 4,493,479. The device disclosed therein is a safety system in which a brake-actuating mechanism is operated in response to a mechanical detector receiving inputs directly from a motor shaft and a drum shaft where the mechanical detector has an output shaft which signals to actuate the brake actuating mechanism when there is a variation in the relative angular volocities between the two shafts to thereby set the brake. For this purpose, Clark utilizes shaft rotation to provide the force for applying the brake or, alternatively, the brake is set by a large force spring controlled by a trigger mechanism.
Among other attempts to provide shaft controlling devices for various applications are those disclosed in Ford U.S. Pat. No. 4,219,107 and Abramham U.S. Pat. No. 2,950,086.
In practice, it has remained to provide a satisfactory shaft relative speed limiting system useful for a variety of systems. More particularly, it has remained to provide a system of the type described for use on aircraft flap actuation systems where a primary drive shaft is used to connect a power drive unit to flap actuators together with a secondary drive shaft. Moreover, it has remained to provide such a system for use on aircraft flap actuation systems utilizing two power drive units each including an electric motor connected by means of a flexible shaft to one side of a torque sensor to normally drive the flaps.
The present invention is directed to overcoming the above stated problems and accomplishing the stated objects.